Fix #299: add support for service readiness notification

This patch adds service readiness notification to support daemons
employing systemd and s6 notification.  Complementing the native
Finit readiness support using PID files that exist already.

The two have slightly different ways of implementing readiness:

 - https://www.freedesktop.org/software/systemd/man/sd_notify.html
 - https://skarnet.org/software/s6/notifywhenup.html

Finit now provides both a NOTIFY_SOCKET environemnt variable, for
systemd, and a way to start s6 daemons with a descriptor argument.

For details on the syntax, see the `service` documentation.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/client.c

@@ -69,10 +69,15 @@ int client_disconnect(void)
 	return rc;
 }
 
-int client_send(struct init_request *rq, ssize_t len)
+/* Only valid after client_connect() */
+int client_socket(void)
+{
+	return sd;
+}
+
+int client_request(struct init_request *rq, ssize_t len)
 {
 	struct pollfd pfd = { 0 };
-	int result = 255;
 	int rc;
 
 	if (client_connect() == -1)
@@ -90,6 +95,16 @@ int client_send(struct init_request *rq, ssize_t len)
 		return -1;
 	}
 
+	/*
+	 * Exception, only command that don't need to wait for a
+	 * response from Finit.  Registers the client socket as a
+	 * readiness notification socket with Finit
+	 */
+	if (rq->cmd == INIT_CMD_NOTIFY_SOCKET) {
+		rq->cmd = INIT_CMD_ACK;
+		return 0;
+	}
+
 	pfd.fd = sd;
 	pfd.events = POLLIN | POLLERR | POLLHUP;
 	if ((rc = poll(&pfd, 1, 2000)) <= 0) {
@@ -108,13 +123,32 @@ int client_send(struct init_request *rq, ssize_t len)
 	}
 
 	if (rq->cmd == INIT_CMD_NACK)
-		result = 1;
-	else
-		result = 0;
-exit:
+		return -1;
+
+	return 0;
+}
+
+int client_send(struct init_request *rq, ssize_t len)
+{
+	int rc = 0;
+
+	if (client_request(rq, len))
+		rc = 255;
+
 	client_disconnect();
 
-	return result;
+	return rc;
+}
+
+int client_command(int cmd)
+{
+	struct init_request rq = {
+		.magic = INIT_MAGIC,
+		.runlevel = getpid(),
+		.cmd = cmd,
+	};
+
+	return client_request(&rq, sizeof(rq));
 }
 
 svc_t *client_svc_iterator(int first)
@@ -151,7 +185,7 @@ error:
 	return NULL;
 }
 
-static svc_t *do_cmd(int cmd, const char *arg)
+static svc_t *do_find(int cmd, const char *arg)
 {
 	struct init_request rq = {
 		.magic = INIT_MAGIC,
@@ -182,13 +216,12 @@ error:
 
 svc_t *client_svc_find(const char *arg)
 {
-	return do_cmd(INIT_CMD_SVC_FIND, arg);
+	return do_find(INIT_CMD_SVC_FIND, arg);
 }
 
-
 svc_t *client_svc_find_by_cond(const char *arg)
 {
-	return do_cmd(INIT_CMD_SVC_FIND_BYC, arg);
+	return do_find(INIT_CMD_SVC_FIND_BYC, arg);
 }
 
 /**